Modern motor vehicles comprise, in addition to their basic functionality, various control and regulation programs, which assist the driver in hazardous situations, e.g. in the event of heavy braking processes, during spinning processes or in other situations in which the vehicle threatens to become unstable, and act to impose stabilization of the vehicle. Here in particular the anti-lock braking system (ABS), the traction control (ASR) and the electronic stability program (ESP) have been established and proven. All three programs or systems intervene actively in the control of the vehicle in situations in which the stability of the vehicle is at risk. E.g. if the ABS detects the locking of one or more wheels during a braking process, the adhesion of said wheels to the ground is improved again by the required pressure decrease and pressure build-up in the brakes. By means of said measures, the braking distance is not necessarily shorter, but the vehicle is in a stable state during the braking process. Equally, spinning of the wheels, e.g. on a slippery surface, can be detected with the aid of the traction control. Here too, the adhesion of the wheels on the road or on the ground is improved by suitable countermeasures. Stabilization of the automobile in situations in which the automobile is at risk of spinning and leaving the highway, e.g. during fast travel round a bend, can be achieved by the ESP, the vehicle being kept on track by means of short braking pulses.
An important component of the ESP and ASR systems is the engine braking control (MSR), which prevents wheel slip of the driven wheels, in particular on a smooth highway, if the driver abruptly takes his foot off the gas or the clutch is engaged too quickly when downshifting. The controller of the engine or the engine controller then temporarily increases the torque at the demand of the ESP controller in order to keep the vehicle stable. In the case of hybrid vehicles, instead of an engine controller a hybrid manager is usually used, which controls both the internal combustion engine and also the electric motor. Furthermore, powertrain managers are also used. Within the scope of this claim, engine controller in each case means the corresponding apparatus used in the vehicle.
A parameter that plays an important part in all said situations is the so-called slip of the respective wheel, which characterizes the ratio of the actual distance travelled per wheel revolution and the actual wheel circumference. Slip can also be characterized as the ratio of the revolution rate of a driven wheel to that of a (hypothetical) undriven and therefore positively idling wheel.
Whereas a small amount of slip is necessary so that the vehicle can primarily be driven by the wheels, a large amount of slip usually characterizes situations in which the vehicle is unstable. Thus e.g. locking of the wheels is characterized by a large amount of braking slip and spinning of the wheels is characterized by a large amount of drive slip.
For many control mechanisms, which are intended to stabilize the vehicle, the knowledge of the actual vehicle speed is of advantage or is necessary. Said parameter is not determined or cannot be easily determined directly or indirectly in normal systems. It can, however, be determined indirectly, e.g. by means of signals that are determined from wheel speed sensors, which are associated with the respective wheels. If the vehicle speed or vehicle reference speed is known, slip can be detected by means of a comparison with the rotational speeds of the individual wheels.
A known technical object to be achieved therefore consists of determining the vehicle speed from the information provided by the wheel speed sensors in a very reliable manner—the individual wheels can enter into slip. For vehicles with only one driven axle, the signals of the wheel speed sensors associated with the non-driven wheels are particularly advantageously suitable.
This manner of determining the vehicle speed is not easily possible for vehicles in which all wheels are driven, i.e. for all-wheel drive vehicles. Especially with these vehicles, however, it is important to detect a reduction of the reference speed or the vehicle reference speed. A reduction here is understood to be the process by which the determined vehicle reference speed continuously deviates further from the actual vehicle speed. This is in particular in the direction in which the determined vehicle reference speed is lower than the actual vehicle reference speed. A method for the detection of an incorrect vehicle reference speed is known from DE 199 39 979 A1, whereby one or more wheels are decoupled from the drive as required, the detection being carried out with reference to the running properties of the suitably decoupled wheel or wheels.
The detection of the reduction of the reference speed in the context of the technical capabilities of electronic brake systems is especially important for all-wheel drive vehicles with a rigid longitudinal differential (Torsen differential) when coasting or even rigid all-wheel drives or even for hybrid vehicles. The detection of the reduction is especially important in vehicles of this type with large internal combustion engines, with which a large drag torque can be generated, as well as for hybrid vehicles with an electric motor which is used as a generator during braking processes and in this way sometimes generates very strong regeneration torque. A large drag torque occurs e.g. when the driver suddenly takes his foot off the gas or suddenly engages a significantly lower gear. In the presence of such a drag torque or regeneration torque it can occur that all wheels are drawn into slip in synchronism at low coefficients of friction by the drag torque. This means that the wheels turn slower in practice and thus roll over a shorter distance than is traversed by the vehicle.
The occurrence of slip can take place with low dynamics, so that e.g. the ESP controller assumes a normal actual or real deceleration of the vehicle. Because the ESP controller starts from a regular or normal deceleration, no measures for stabilizing the vehicle are initiated. The reduction of the vehicle reference speed, which in the above described cases is a gradual process, then leads to a completely unstable vehicle. Therefore this is especially also the case because the cornering forces of the wheels decrease rapidly for increasing slip.
In these cases the vehicle can hardly be controlled or cannot be controlled at all and usually begins to spin uncontrollably. The problems mentioned occur as soon as the wheel drag torques are greater than the coefficient of friction between the highway and tires that allows this. This can typically occur on black ice. However, for larger or increasing drag torques or regeneration torques, such a process is also conceivable on highways with different surfaces.
A reference decrease or the decrease of the vehicle reference speed was previously unknown or only inadequately known for the above-mentioned vehicle types. The necessity of such detection is, however, strongly increased by the modern developments of hybrid vehicles and all-wheel drive vehicles.
The object of the invention is to provide a method for the detection and correction of the vehicle reference speed, in particular in the event of its decrease by drag torques and/or recovery torques, that on the one hand enables extremely reliable determination of the vehicle reference speed as well as the introduction of corrective measures as soon as a decrease has been detected, and on the other hand can make use of normally constructed components and control and regulation routines. Furthermore, an associated vehicle system is to be specified.